Population size affects genetic diversity and fine-scale spatial genetic structure in the clonal distylous herb Menyanthes trifoliata

tSmall populations often show a low degree of genetic diversity. In distylous clonal plants, genetic diversitymay also be influenced by other factors, such as floral morph aggregation, clonality and spatial geneticstructure. Nevertheless, the relationships between population size, genetic diversity and spatial geneticstructure are poorly understood. We tested for the effects of population size on genetic diversity andfine-scale spatial genetic structure (FSGS) in the clonal, distylous aquatic herb Menyanthes trifoliata. Toinvestigate genetic diversity and structure, we used microsatellite markers and genotyped a total of 510ramets from nine populations in southern Belgium. The populations showed a relatively high degreeof genetic diversity (HEc= 0.80; A[12] = 4.45), even if both expected heterozygosity and allelic richnesssignificantly decreased with decreasing population size (genet numbers). Populations were significantlygenetically differentiated (FST= 0.14), indicating reduced gene flow among populations. Morph ratio forgenets did not deviate from 1:1. The phalanx type of clonal growth dominated in the studied populations.Ramet FSGS (which was strongly affected by clonality) was significant in all populations. Sp statisticsignificantly decreased with increasing population size, indicating higher FSGS in smaller populations.Consequently, small populations may suffer from a higher rate of geitonogamous pollination (betweenramets of the same genet). Since M. trifoliata is a strongly self-incompatible herb, no inbred seeds canbe produced from self- (and geitonogamous) pollination. In this case, reproductive success of small andmore spatially structured populations of M. trifoliata may be reduced due to clogging of stigmas withincompatible pollen. We conclude that understanding genetic diversity and fine-scale clonal structure iscrucial for conservation of self-incompatible plant species with extensive clonal propagation.